Electromagnetic trip device

ABSTRACT

An electromagnetic trip device for a circuit breaker is provided. The electromagnetic trip device includes a current path having a conductor guiding an electric current through at least one contact point formed between a fixed and at least one movable contact piece. A contact lever carries the at least one movable contact piece. The contact lever is mounted to be movable between a closed contact position and an open contact position. The electromagnetic trip device also includes a magnetic circuit with an air-gap. The movable contact lever is at least partially located in the air-gap. The electric current flows through the contact lever in a direction substantially perpendicular to the orientation of magnetic flux in the air-gap.

RELATED APPLICATION

This application claims priority as a continuation application under 35 U.S.C. §120 to PCT/EP 2010/002893, which was filed as an International Application on May 12, 2010 designating the U.S., and which claims priority to European Application 09006577.2 filed in Europe on May 15, 2009. The entire contents of these applications are hereby incorporated by reference in their entireties.

FIELD

The present disclosure relates to an electromagnetic trip device for a circuit breaker, and to an installation switching device with such an electromagnetic tripping device.

BACKGROUND INFORMATION

Electromagnetic trip and installation switching devices of the kind mentioned above are usually electro-mechanical devices. The point of contact includes a fixed contact member and a movable contact member which is held by a movable contact arm or contact bridge. In the closed position, the movable contact member is pressed against the fixed contact member influenced by the force of a contact spring.

Such trip devices and installation switching devices can also include a mechanical gear mechanism with a latch and a spring force based energy storage assembly.

In case of a tripping condition, a tripping device is known to act on the latch, which then releases the energy from the energy storage so that the gear mechanism can act upon the contact lever or contact bridge in order to open the point of contact. Known tripping devices, are for example, thermal tripping devices based on bimetal or thermal shape memory alloys, short circuit tripping devices based on a fast moving electromagnetic armature, and residual current tripping devices based on an integrating current transformer coupled to an electromagnetic armature.

An electromagnetic trip device for a circuit breaker is described, for example, in WO 2006 117 097 A2. The trip device has a yoke, a coil, a magnetic core and a magnetic armature, where the armature is coupled to a pin jutting out from the device in case of a short circuit current. When jutting out, the pin acts on a movable contact lever for directly opening a contact point. Due to the various mechanical pieces involved, each having a non-neglectable mechanical inertia, the delay time between occurrence of the short circuit current and the opening of the contact point cannot fall below a lower limit in the range of several milliseconds.

The known mechanical setup of trip devices and of the kind mentioned leads to limitations when it comes to tripping and switching speed, due to the inertia of the masses of the levers and other gear components involved.

In a range of low voltage applications, known electromechanical actuators are too slow. For example, breakers in low voltage systems (e.g., power supply for telecom) need to be fast operating in order to provide selectivity and protection, faster than is today possible with the known electro-mechanical devices. As a result, presently the only alternative is an electronic switch. This, however, has the disadvantage of missing galvanic separation, which can only be achieved in mechanically operating circuit breakers.

SUMMARY

An exemplary embodiment of the present disclosure provides an electromagnetic trip device for a circuit breaker. The exemplary electromagnetic trip device includes a current path having a conductor guiding an electric current through at least one contact point formed between a fixed contact piece and a movable contact piece. The exemplary electromagnetic trip device also includes a contact lever carrying the at least one movable contact piece, the contact lever being mounted to be movable between a closed contact position and an open contact position. The exemplary electromagnetic trip device includes a magnetic circuit including an air-gap in which the movable contact lever is at least partially located. The electric current flows through the contact lever so that a magnetic field generated by the current flowing through the contact lever directly interacts with a magnetic field within the air-gap. The air gap has a width of less than 1 mm.

An exemplary embodiment of the present disclosure provides installation switching device, which includes an electromagnetic tripping device, a current path, at least two contact members, and a contact point in the current path. The contact point is formed by at least two contact members. At least one of the contact members is mounted on a movable contact lever. The contact members are configured to be separated in a tripping condition under the influence of the electromagnetic tripping device. The electromagnetic trip device includes a magnetic circuit including an air-gap. The movable contact lever is at least partially located in the air-gap, and current path flows through the contact lever in a direction substantially perpendicular to the orientation of magnetic flux in the air-gap.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional refinements, advantages and features of the present disclosure are described in more detail below with reference to exemplary embodiments illustrated in the drawings, in which:

FIG. 1 a shows a schematic longitudinal-sectional view of an electromagnetic tripping device according to an exemplary embodiment of the present disclosure;

FIG. 1 b shows a schematic top view of the exemplary embodiment shown in FIG. 1 a;

FIG. 2 shows a schematic cross-sectional side view of the exemplary embodiment shown in FIG. 1 a;

FIG. 3 a shows a schematic longitudinal-sectional view of an electromagnetic tripping device according to an exemplary embodiment of the present disclosure;

FIG. 3 b shows a schematic top view of the exemplary embodiment shown in FIG. 3 a;

FIG. 4 shows a schematic cross-sectional side view of the exemplary embodiment shown in FIG. 3 a; and

FIG. 5 shows a scheme of an installation switching device with an electromagnetic trip device according to the exemplary embodiment shown in FIG. 1 a.

DETAILED DESCRIPTION

Exemplary embodiments of the present disclosure provide an electromagnetic tripping device which operates much faster than those known in the art. Exemplary embodiments of the present disclosure also provide an installation switching device having at least two contact members which are configured to be separated in a tripping condition under the influence of such an electromagnetic tripping device.

For instance, exemplary embodiments of the present disclosure provide ways and means for designing an electromagnetic tripping device in such a way that its tripping and switching speed is much faster than with known devices, and which still provides galvanic separation of the contacts in the open contact position.

Exemplary embodiments of the present disclosure also provide an installation switching device with a very fast electromagnetic tripping device.

In accordance with an exemplary embodiment of the present disclosure, the electromagnetic trip device includes a magnetic circuit with an air-gap, and a movable contact lever is at least partially located in the air-gap. Electric current flows through the contact lever, so that the magnetic field generated by the current flowing through the contact lever directly interacts with the magnetic field within the air-gap.

According to an exemplary embodiment of the present disclosure, the current through the contact lever flows in a direction substantially perpendicular to the orientation of the magnetic flux in the air-gap.

An electromagnetic tripping device according to an exemplary embodiment of the present disclosure provides that there is a direct interaction between the magnetic field or flux of the magnetic circuit and the contact lever. By contrast, in known electromagnetic tripping devices, there is only an indirect interaction between the contact lever and the magnetic circuit. The direct interaction in known tripping devices is between the magnetic circuit and a pin which is accelerated as a result of such interaction and which then hits the contact lever to open the contact point. In accordance with an exemplary embodiment of the present disclosure, there is no mechanical device like a pin between the magnetic circuit and the contact lever. As a result, there is no delay time that would be caused by the mechanical inertia of such intermediate mechanical components.

The force acting on the contact lever results from the well known Lorentz force which acts on a moving electric charge in a magnetic field, thus on a conductor in a magnetic field when an electric current flows through it. In case the orientation of the flux of the magnetic field is perpendicular to the current, the Lorentz force has its highest value and its direction is then perpendicular both to the orientation of the magnetic flux and the current flow, where the orientation is given by the well known “three-fingers-rule”.

Thus, in case the current in the current path of a circuit breaker including an electromagnetic tripping device according to the present disclosure is a short circuit current exceeding a determinable threshold value, the Lorentz force immediately pushes the contact lever out of the air gap, and as at least one movable contact piece is fixed to the contact lever, this will thus be torn away from the fixed contact piece to thereby open the contact point and interrupt the short circuit current.

The only inertia involved now is that of the contact lever, and this can be made very small if the contact lever is designed in a way such that its moment of inertia in the direction of the movement is small. According to an exemplary embodiment of the present disclosure, this can be achieved by designing the contact lever in such a way that its portion which interacts within the air gap with the magnetic field of the air gap has a flat but high cross-section, so that a large cross-section area is achieved with a small thickness. The delay time when opening the contact point in a short circuit event thus is very short, much shorter than can be achieved with known magnetic tripping devices.

The faster the opening of the contact point happens, the less the power dissipation in the installation switching device—which has its origin in the arc between the contact members when opening the contact point—so that an electromagnetic tripping device according to the present disclosure can advantageously be used for high current limiting values, such as, for example, 63 kA.

According to an exemplary embodiment of the present disclosure, the contact lever is configured as a contact bridge, carrying two movable contact pieces and connecting two contact points. The contact force can be achieved by contact springs pushing the contact pieces together, and the resetting force of which determines the threshold current at which the tripping device will interrupt the current path.

According to an exemplary embodiment of the present disclosure, the magnetic circuit includes a magnetic core made of a ferromagnetic material with an air gap. The magnetic flux in the air gap is oriented substantially perpendicular to the air-gap walls, and the contact lever has a portion in the shape of a longitudinally extended flat conducting bar, where this portion is configured so that it can slide within the air gap parallel to the walls of the air gap.

According to an exemplary embodiment of the present disclosure, the latter portion of the contact lever has a flat but high cross-section, so that a large cross-section area is achieved with a small thickness.

According to an exemplary embodiment of the present disclosure, the conductor forms at least one winding around the magnetic core so that the electric current in the current path generates the magnetic flux in the air gap.

According to an exemplary embodiment of the present disclosure, the magnetic circuit includes a permanent magnet providing the magnetic flux within the air gap.

According to an exemplary embodiment of the present disclosure, the magnetic circuit additionally includes a permanent magnet so that both the electric current in the current path and the permanent magnet generate the magnetic flux in the air gap.

In accordance with an exemplary embodiment of the present disclosure, the magnetic core is made up of laminated metal sheets.

In accordance with an exemplary embodiment of an installation switching device according to the present disclosure, the electromagnetic trip device includes a magnetic circuit with an air-gap and the movable contact lever is at least partially located in the air-gap. The current path flows through the contact lever in a direction substantially perpendicular to the orientation of the magnetic flux in the air-gap.

In accordance with an exemplary embodiment of the present disclosure, the anode-cathode voltage of an arc across a contact system is used to limit the current in a low voltage network within approx. 0.1 msec time. In order to create a contact lift based only on the system current I which is flowing through the installation switching device and which in case of a short circuit will rise very fast to approx. 1 kA, the following solution is proposed. A one—or two—turn winding through the laminated magnetic core 11 is creating a flux (F) φ passing across an air gap 10 where the moving part of the contact system, here a contact bridge 8, is at rest in the normal condition. When the current rises steeply, a lifting force F is created proportional to the flux density and the current I. The moving contact will lift and at a contact gap of about 0.01-0.05 mm the anode—cathode voltage of the arc is reached. This will limit the current I to approx. 1 kA. To enable contact lift in the required short time a very narrow gap 10 is realized, about 0.6 mm, to reach a high flux Φ density already in the low current phase. Furthermore the mass of the moving contact is held very low, by making the portion 12 of the contact lever 8 which is sliding within the air gap 10 flat.

Exemplary embodiments of the present disclosure are further described below with reference to the drawings.

FIG. 1 a shows, in a longitudinal sectional view, a magnetic circuit 9 having a magnetic core 11.

The magnetic core 11 can be made of laminated metal sheets, like a transformer core. A conductor 13 carrying the current path leads through the central opening 15 of the magnetic core 11, forming a winding around the top portion of the core 11 and coming down to the base part of the core 11. There, the magnetic core 11 has a narrow gap 10, for example, about 0.6 mm in width.

In front of the magnetic core 11 the conductor 13 ends, its free end carrying a fixed contact piece 4. Opposite the magnetic core the current path continues by a second conductor 18, carrying on its free end, facing the magnetic core 11, a fixed contact piece 6.

The current path is closed by a contact bridge or contact lever 8. The contact lever 8 has on its free ends contact portions 16, 17, which are connected by a central portion 12.

The contact portion 16 carries a movable contact piece 5, and the contact portion 17 carries a movable contact piece 7. The movable contact piece 5 is pressed against the fixed contact piece 4 by a first contact spring 19 forming a first contact point 3, and the movable contact piece 7 is pressed against the fixed contact piece 6 by a second contact spring 20, forming a second contact point 2.

The central portion 12 is a thin but high metal sheet. For example, the central portion 12 can be about 0.5 mm thick leaving a very narrow free air-gap of 0.1 mm. Its height is nearly the height of the gap 10. Thus, the central portion 12 nearly completely fills the gap 10, leaving only a very narrow free air gap. The central portion 12 is mounted so that it can glide up and down in parallel to the walls of the gap 10. The small free air gap results in a high flux density of the magnetic flux Φ in the air gap which then results in a high force being applied onto the contact lever 8.

FIG. 1 b shows a longitudinal-sectional top view according to section BB in FIG. 1 a, and FIG. 2 shows a cross-sectional side view according to section AA in FIG. 1. FIG. 1 b shows the transition between the broad contact portions 16, 17 and the thin but high central portion 12 of the contact lever or contact bridge 8.

The function of the arrangement shown in FIGS. 1 a, 1 b and 2 is as follows. The current through the current path flows through the first conductor 13 in direction of arrows I. The current passes through the contact bridge 8 via the two contact points 2 and 3, and leaves through the second conductor piece 18. The winding of the first conductor around the magnetic core 11, in the upper part of FIG. 1, causes a magnetic flux in the magnetic core which, in FIG. 1, comes out of and substantially perpendicular to the plane of projection in the lower part, and enters the plane of projection in the upper part of the magnetic core 11 visible in FIG. 1 a. In the air gap 10, the magnetic flux created by the current is thus directed substantially perpendicular to the direction of the current flow I in the central portion 12 of the contact bridge 8. As a result, a Lorentz force F acts on the contact bridge 8 pulling it vertically upwards into the central opening 15 of the magnetic core 11. The Lorentz force acts against the repelling force of the contact springs 19, 20. An increasing current flow increases the Lorentz force, a result of two cumulating effects: the magnetic flux is increased, and the current itself is increased. The absolute value of the Lorentz force is proportional to the product of current times flux, so the Lorentz force increases strongly on an increase of current flow through the current path. In case of a short circuit current, the current flow increases quickly. As soon as the Lorentz force F surpasses the repelling force of the contact springs 19, 20, the contact bridge 8 is torn away in the upward direction, separating the movable contact pieces 5, 7 from the fixed contact pieces 6, 8. A short circuit interruption of the current path has been achieved.

An additional force acting on the contact lever 8 and attracting it into the central opening 15 results from the parallel direction of the current flowing through the portion 13′ of the conductor piece 13 and the portion 12 of the contact lever 8. It is known that parallel conductors with current flowing through both of them in the same direction attract each other due to a weakening of the magnetic field in between them.

The fact that the portion 12 of the contact bridge 8 within the air gap 10 is made flat might lead to a reduction in the conducting area of the moving contact and hence the rated current might become too low. For this reason, the portion 12 is made flat but high, as can be seen, for example, in FIG. 2 in the cross-sectional view, thus achieving a sufficient cross-sectional area.

An additional solution to this problem is that the air gap 10 of the core 11 acts as a cooling agent along the conducting area of the contact bridge 8. With an air gap of, for example, 0.6 mm and the thickness of the moving contact of 0.5 mm, the thermal conduction takes place through an air gap 10 of only 0.05 mm. The cooling becomes very effective and a current density of at least 20 A/mm², for example, is possible. This solution has the advantage of both extremely fast current limitation capability without restricting rated current capability.

The approach described here is to decrease the air gap 10 to a minimum, here 0.6 mm, for example, and still handle considerable currents due to the effective cooling in the air gap 10. The cooling effect allows the height of the portion 12 to be limited and thus limiting the overall dimensions of the device.

Another advantage is the mechanical stabilization of the portion 12. The walls of the air gap 10 act as support for the portion 12, avoiding that it gets a permanent deflection by the forces applied by the magnetic field against the springs, the air gap 10 thus becoming the sliding support for the portion 12.

In accordance with an exemplary embodiment of the trip device of the present disclosure, the anode-cathode voltage is the main limiting voltage, whereas in known solutions the arc voltage has been the main limiting voltage. In an example, for a nominal voltage of 60 V, the anode—cathode—voltage is created nearly instantly, very fast, resulting in a high limiting speed of less than 0.1 msec.

The arc extinguishing voltage is the sum of anode-cathode voltage and the voltage drop across the arc length. In accordance with an exemplary embodiment, the trip device according to the present disclosure mainly uses the immediate anode-cathode voltage and needs not wait for the elongation of the arc.

When opening a contact and drawing apart the contact pieces, an arc extinguishing voltage is generated and is oriented contrary to the supply voltage. The current flow stops as soon as the arc extinguishing voltage exceeds the supply voltage.

The goal of any interrupter device is to minimize the time until the arc extinguishing voltage exceeds the supply voltage so that the current flow will be interrupted as fast as possible.

In known devices being applied at high supply voltages of, for example, 230 V or more, the anode-cathode voltage created at contact opening is not sufficient to limit the current owing to high supply voltage. That is why such devices usually have an additional arc extinguishing chamber with a staple of arc extinguishing plates, and the arc generated when opening the contacts is lead into the arc extinguishing chamber.

When pulling apart the contact pieces in such a high voltage application, what happens is usually the following.

Upon first opening of the contacts, an anode-cathode-voltage is generated at the contact pieces. However, the anode-cathode voltage created at contact opening is not sufficient to limit the current owing to high supply voltage, the anode-cathode-voltage being less than the supply voltage.

After creation of the first anode-cathode voltage, contacts have to separate some millimeters more until the arc is able to move off the contacts. The arc then has to travel and elongate before entering into an extinguishing chamber. In the chamber, the arc then has to split into many anode-cathode voltage steps, each one appearing at one of the arc extinguishing plates within the arc extinguishing chamber. This takes additional time and is the reason why there is a certain minimum time for deleting current flow when opening contacts in a high voltage application.

In other words, the arc moves into the arc extinguishing chamber and then between every plate of the chamber an anode-cathode voltage is created. These anode-cathode voltages are in series and if the sum of the voltages is higher than the supply voltage, the arc is extinguished. The arc needs some time to move into the arc chamber.

The trip device according to the present disclosure can be designed for a low supply voltage, for example, a maximum supply voltage of 60 V. In this case, the two anode-cathode voltages at the two contacts will exceed the supply voltage immediately and the arc will extinguish. There is no need for an arc to move into an arc chamber and due to this, the arc extinguishing according to the present disclosure is very fast.

An exemplary embodiment of the present disclosure introduces a permanent magnet 14 in the core in combination with the winding or separately replacing the winding. FIGS. 3 a, 3 b and 4 show the exemplary embodiment where a permanent magnet 14 is used alone to create the magnetic flux in the air gap 10. The same or similar or similarly functioning elements or sub-modules in FIGS. 3 a, 3 b and 4 have the same reference signs as those in FIGS. 1 a, 1 b and 2.

FIGS. 3 a, 3 b and 4 show an exemplary embodiment where the contact lever 8 is pushed out of the gap 10, downwards in FIGS. 3 a and 4. This is achieved by arranging the permanent magnet in such a way that the direction of the magnetic flux Φ is opposite to that as shown in the embodiments of FIGS. 1 a, and 2. The change of direction of magnetic flux can, of course, also in embodiments with current-induced magnetic flux be achieved by adapting the sense of turning of the coil made up by the conductor 13 accordingly.

FIG. 5 shows, in a schematic view, an installation switching device 21, for example, a circuit breaker, having a housing 22 made of an insulating material. The switching device has on one side 23 a first connection terminal 24 for connection of a conductor, and on the opposite side 25 a second connection terminal 26 for connection of another conductor. A current path is flowing between the two connection terminals 24, 26 through the device 21. An electromagnetic trip device 1 as shown and described above with reference to FIG. 1 a, 1 b, 2 is located inside the device 21 and part of the current path. In case of a short circuit current flowing through the device 21, the electromagnetic trip device 1 will interrupt the current flowing through the device 21 as described above.

The present disclosure shall not be limited to the exemplary embodiments shown, but each equivalent shall certainly be included within the range of protection of this specification.

One example for a variation that shall be covered by the present disclosure is a version where a permanent magnet is combined with magnetization through the current flow.

Another variant is to use a rotatably mounted contact lever, which would make a rotating movement within the gap, having a single contact point at its free end. Its rotating axis would be at the transition point to the free end of conductor 13.

It will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the fore-going description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein.

List of reference signs:  1 Electromagnetic trip device  2 Second contact point  3 First contact point  4 fixed contact piece  5 Movable contact piece  6 fixed contact piece  7 Movable contact piece  8 Contact lever  9 Magnetic circuit 10 Air-gap 11 Magnetic core 12 portion 13 First conductor 13′ Portion of conductor 14 Permanent magnet 15 Central opening 16 Contact portion 17 Contact portion 18 Second conductor 19 First contact spring 20 Second contact spring 21 Installation switching device 22 Housing 23 First side 24 Connection terminal 25 Second side 26 Connection terminal Φ Magnetic flux F Force I current 

1. An electromagnetic trip device for a circuit breaker, comprising: a current path including a conductor guiding an electric current through at least one contact point formed between a fixed contact piece and a movable contact piece; a movable contact lever carrying the at least one movable contact piece, the contact lever being mounted to be movable between a closed contact position and an open contact position; and a magnetic circuit including an air-gap in which the movable contact lever is at least partially located, wherein the electric current flows through the contact lever, the current path extending through the air gap, so that a magnetic field generated by the current flowing through the contact lever directly interacts with a magnetic field within the air-gap to generate a Lorentz-type repelling force on the contact lever, and wherein a central portion of the contact lever is configured to be a thin but high metal sheet, which is configured to glide in the air gap with its broad sides in a parallel manner adjacent to side walls of the air gap.
 2. The electromagnetic trip device according to claim 1, wherein the current through the contact lever flows in a direction substantially perpendicular to the orientation of magnetic flux in the air-gap.
 3. An installation switching device comprising: the electromagnetic tripping device according to claim
 2. 4. The electromagnetic trip device according to claim 1, wherein the contact lever is configured as a contact bridge to carry two movable contact pieces and connect two contact points.
 5. An installation switching device comprising: the electromagnetic tripping device according to claim
 4. 6. An installation switching device comprising: the electromagnetic tripping device according to claim
 4. 7. The electromagnetic trip device according to claim 1, wherein: the magnetic circuit comprises a magnetic core made of a ferromagnetic material with an air gap; magnetic flux in the air gap is oriented substantially perpendicular to walls of the air-gap; the contact lever includes a portion in the shape of a longitudinally extended flat conducting bar, the portion being configured to slide within the air gap parallel to the walls of the air gap.
 8. The electromagnetic trip device according to claim 7, wherein the portion of the contact lever has a flat but high cross-section, so that a large cross-section area is achieved with a small thickness.
 9. An installation switching device comprising: the electromagnetic tripping device according to claim
 8. 10. The electromagnetic trip device according to claim 7, wherein the conductor forms at least one winding around the magnetic core so that the electric current in the current path generates the magnetic flux in the air gap.
 11. The electromagnetic trip device according to claim 10, wherein the magnetic circuit comprises a permanent magnet so that both the electric current in the current path and the permanent magnet generate the magnetic flux in the air gap.
 12. An installation switching device comprising: the electromagnetic tripping device according to claim
 11. 13. An installation switching device comprising: the electromagnetic tripping device according to claim
 10. 14. The electromagnetic trip device according to claim 7, wherein the magnetic circuit comprises a permanent magnet providing the magnetic flux within the air gap.
 15. An installation switching device comprising: the electromagnetic tripping device according to claim
 14. 16. The electromagnetic trip device according to claim 7, wherein the walls of the air-gap are configured to contribute to the mechanical stabilization of the contact lever by forming a sliding support for the portion.
 17. An installation switching device comprising: the electromagnetic tripping device according to claim
 16. 18. An installation switching device comprising: the electromagnetic tripping device according to claim
 7. 19. The electromagnetic trip device according to claim 1, wherein the contact lever is configured such that its moment of inertia in a direction of movement in a direction out of the air gap is small.
 20. An installation switching device comprising: the electromagnetic tripping device according to claim
 19. 21. The electromagnetic trip device according to claim 1, wherein the magnetic circuit comprises a magnetic core made of a ferromagnetic material with an air gap, and wherein the magnetic core is made up of laminated metal sheets.
 22. An installation switching device comprising: the electromagnetic tripping device according to claim
 21. 23. An installation switching device comprising: the electromagnetic tripping device according to claim
 1. 24. The electromagnetic trip device according to claim 1, wherein the air gap has a width of less than 0.8 mm.
 25. The electromagnetic trip device according to claim 1, wherein the air gap has a width of about 0.6 mm.
 26. The electromagnetic tripping device according to claim 1, wherein the thin but high metal sheet is configured to be guided in the air gap to provide mechanical stabilization of the thin metal sheet a cooling effect, by supporting cooling of the contact lever in case a high short circuit flows therethrough. 